1. Field of the Invention
This invention relates to an electron beam welding method and, more specifically, to an electron beam welding method that uses a modified electron beam pattern.
2. Description of the Prior Art
The electron beam welding method is characterized by the fact that in view of the high power density there can be obtained a weld bead having a narrow width and a deep penetration.
On the other hand, the behavior of the molten metal becomes very complicated, since the welding must be carried out while forming a narrow elongated electron beam cavity for obtaining a deep panetration, and this results in the formation of its peculiar defects.
Specifically, in the electron beam welding method, a beam cavity is formed at the front side in the welding direction, while the molten metal accumulates at the rear side. The molten metal is ejected externally of the beam cavity by the pressure of the metal vapor, followed by an inflow of the molten metal from the outside of the beam cavity. Now, if this ejection and inflow of the molten metal is repeated periodically in a suitable manner, the welding can be favorably carried out.
For example, if the power density of the beam at the root portion is low or the opening of the beam cavity at the surface of the material being welded is small, the molten metal does not flow out properly. Hence, the already solidified wall is melted by the accumulated molten metal in the beam cavity to cause the formation of depressions at intermediate points of the beam cavity, with the consequence that voids, i.e., porosities, or various solidification cracks are formed.
On the other hand, if the energy density of the beam at the root portion is too high, sharp spikes are formed at the root portion which become the cause of poor melt bonding, i.e., coldshuts, as well as the formation of voids, i.e. root porosities. Hence, there has been the drawback that the strength of the weld can not be increased.
As means of resolving the aforesaid porosities and formation of solidification cracks, there has been suggested a method of welding which involves carrying out the welding while oscillating the electron beam in the welding direction or a direction perpendicular thereto. In the case of the method where the electron beam is oscillated in the welding direction, there are, however, such drawbacks as that violent sputtering takes place as a result of the electron beam reheating the molten metal of high temperature and that sharp spikes are formed at the root portion. On the other hand, in the case of the method in which the welding is performed while oscillating the electron beam in a direction perpendicular to the welding direction, there is the drawback that the bead width spreads and the penetration cannot be made deep.
The electron beam welding method has however the advantage that, as compared with the other welding methods, the diameter of the heat source is small and the power density is remarkably high, with the consequence that it is a welding method of high efficiency that can weld thick plates in a single step.